The overall scheme for reducing distortion by adding an error signal to the delayed output of a nonlinear device resembles electrical feedforward schemes, and operates under the same constraints in terms of required phase and amplitude marching in the signal and error paths. The difference is the use of light, and optical components, rather than electricity. A second difference is the system is used to produce a modulated (optical) signal rather than amplify an existing modulated (optical) signal.
A "quasi-feedforward" approach using two modulated optical devices with identical transfer functions has been previously demonstrated with light emitting diodes (Strauss, Electronics Letters 13, 158 (1977)).
A feedforward approach using two identical laser diodes has been demonstrated, where the main signal and error signal are sent over two separate fibers to two separate receivers, and then combined in a single electrical output (Franckart, ECOC 83, European Conference On Optical Communications, Elsevier Science Publishers, 1983, pp. 347-350).
Compared to the present invention, this use of two identical sources increases the received noise significantly. Two laser diodes used this way are not capable of obtaining the required 55 dB carrier to noise ratio with 80 channels of C.A.T.V. signals in a 550 MHz bandwidth, with the required -65 dB composite distortion products. The use of two diode pumped Nd-YAG or Nd-YLF lasers and two Mach-Zehnder modulators would not be an economical solution.
Also in contrast to the present invention, only one optical output is provided, divided between two optical fibers which must both be present at the receiver to generate the single electrical output.
Additionally, any change in optical length or attenuation of the two fibers will affect the main signal and the error signal unequally, and reduce the distortion cancellation. This is particularly true of optical length, where a relative change of a few inches in a 10 mile link would degrade system performance by 10 dB or more. In contrast, in the present invention, both the main signal and the error signal travel over the same fiber and are affected equally, leading to no system degradation.
A feedforward approach using two identical optical sources, each consisting of a diode laser and a Mach-Zehnder interferometer, has recently been published (Ridder et al., Technical Abstract, Optical Fiber Communication Conference, San Francisco 1990). This exhibits unacceptable noise performance for cable television, due to the low launch power and high noise of diode lasers. It would not be possible, without modification, to implement such a system with diode-pumped high power, low noise Nd-YLF lasers. This is because coherent interference at the optical combiner from two such lasers of the same wavelength would add noise to the signal. This could be avoided by the use of Nd-YAG and Nd-YLF lasers of different wavelengths or the addition of wavelength selective elements to the Nd-YAG cavity, but the use of two diode pumped lasers instead of one significantly increases the cost of the system compared to the invention described here. Again, in contrast to the present invention, only one optical output is provided, the remaining optical power being wasted in a three port combiner.